1. Field of the Invention
This invention relates to the installation of an electronic circuit inside the structure of a tire, allowing, for example, a simple and economical method for identifying or monitoring the tire.
2. Discussion of Background
A transponder integrated circuit installed inside a tire to identify it has already been described in U.S. Pat. No. 4,911,217. This patent explains that it has already been proposed to use identification transponders that are installed inside tires and coupled inductively with the outside. This relates more particularly to the tires for a heavy truck since the cost that could be expected for transponders thus designed was too high to consider an application in the field of tires for passenger vehicles.
U.S. Pat. No. 4,911,217 describes a method for, from an integrated circuit, installing electrodes quite near a metal reinforcement structure inside a tire for a heavy truck to operate from an oscillating electric field rather than a magnetic field as was considered previously. In this way, the costly and troublesome inductive antenna is eliminated, since a capacitive coupling is used between the inquiry device and the integrated circuit installed inside the tire.
This solution, however, has a certain number of drawbacks. The first of these drawbacks is that it requires that the tire be provided with a metal reinforcement structure. This will not always be the case, and in particular the invention as it is described in the cited patent cannot be applied in a tire in which all the reinforcements would be of textile.
Another problem resides in the fact that it is difficult, in the industrial processes, to install the electrodes with great geometric precision during the production of the tire. Now, according to the teaching of that patent, "the difference in spacing for each of the two electrodes relative to a metal reinforcement element of the tire is an essential characteristic of the invention." This will prove certainly difficult to guarantee in an industrial environment during the production of the tire, especially since the installation of electrodes is performed at a time when most of the components of the tire are of raw rubber, i.e., not having a great dimensional stability, nor in general good mechanical characteristics.
Further, the invention described requires devices for identification by reading and writing, or reading only, which are technically complex, quite as complex as in the case of an inductive coupling. The costs for these inquiry devices are too high to assure a large distribution of them. It is relatively easy to have available the required high voltages (one thousand volts peak-to-peak as suggested in the patent, probably much more for inquiry devices allowing read and write), but it is more difficult to receive signals of low level and of high impedance source such as those transmitted by the transponder circuit concealed in the tire, from a capacitive antenna located outside the tire. The difficulties are still greater when it is necessary to operate inside environments that are electrically very noisy, such as production factories or a motor vehicle.
Electroconductive rubber elements placed on or buried in the tire tread or the walls for remaining static electricity are also known by patent application JP 73/057 302.
It is also known by patent FR 2 411 719 to make rubbers having a good mechanical resistance to the action of electric voltage, for the tires equipping vehicles using electric energy to move, such as trains, streetcars or subways.
To date, no use of the electrical characteristics of rubbers has been made to impart to the tires electric or electronic functions in the problem of the identification or of the monitoring of a tire, in use on a vehicle, or during its production.
The electrical conductivity of the rubber compositions used in the production of tires is mainly due to the types and to the proportions of carbon black used to charge the rubber. A tire is a very complex structure, relying in particular on different rubber composition types, which, when they are assembled and vulcanized, impart a fairly uniform appearance to the tire. Actually, the investigations of the applicant have led him to observe considerable differences in electric resistivity in the various rubber elements of the tire, even when nothing is done intentionally to influence the electrical properties of the tires, which is generally the case. FIG. 1 illustrates the valid electrical characteristics for almost all tires produced at present. Several zones in which the rubber compositions are electrically conductive are seen. They include the protective layer of the beads, called "covers" 11 and 12, and the tire tread 13 below.
Covers 11, 12 offer a fairly low electrical resistance value, in general less than 10,000 ohms, while tire tread 13 has an electrical resistance of a value typically less than 500,000 ohms. For all the other rubber elements, the measured electrical resistance is considerably higher, on the order of several M ohms (.gtoreq.10.sup.6 .OMEGA.).
It has been found that the measured electrical resistance virtually did not depend on the distance between the measuring electrodes, at least beyond a certain distance on the order of several centimeters. This is probably due to the fact that, since all the rubber elements of the tire are rotating parts, the electrical conduction is performed by the shortest path between the two electrodes, and also by the longest path, i.e., by two paths associated electrically in parallel. Further, the electrical contact resistance between the electrode and the rubber element probably has a significant value relative to the internal resistance of the rubber element, which is also a balancing factor of the measured resistances, regardless of the locations between which the measurement is made. Further, the metal reinforcement elements of the tire, such as the crown plies and bead wires 14, undoubtedly take part also in the conduction and are also a balancing factor.